1. Field of the Invention
Heretofore, suspension fertilizers produced by the direct ammoniation of merchant-grade wet-process orthophosphoric acids in batch equipment were required to be of low grade because the acids had to be considerably diluted with water (i.e., from about 54% down to about 25-30% P.sub.2 O.sub.5) prior to the ammoniation step to prevent severe thickening caused by a combination of excessive crystallization of monoammonium phosphate (MAP) and metallic impurity gel formation during the ammoniation step. Although pure mineral acids, such as for example, phosphoric acid theoretically have a pH of about 0, the commercial acids normally utilized in fertilizer production have initial pH's ranging anywhere between about 0.1 to about 0.5. These higher than theoretical initial or "bottom" pH's are due, at least in part, to the congeneric impurities in such impure acids. Also, heavy ammonia losses occurred if the acids were not of the proper dilution because the slurry would "set up" in the pH range of 2 to 4. Suspension fertilizers with such low analysis (such as 8-24-0) have a very distinct economic disadvantage as compared with higher analysis products because the costs of handling, freight, storage, and application are higher per unit of plant nutrient.
In order to obtain higher grades, the phosphoric acid had to be added to a heel from a previous batch equal to at least 33 percent of the total batch volume. The heel increased the pH and avoided severe complications caused by the excessive crystallization of monoammonium phosphate and metallic impurity gel formation during the ammoniation step. The use of a heel in this large volume percentage (33% or higher) of the total batch volume would be detrimental for batch-plant sizing and cycle times.
2. Description of the Prior Art
The art of producing fluid fertilizers, both liquids and suspensions, is well known and fully described in the literature.
The most common method of producing both liquid and suspension fertilizers containing nitrogen and phosphate is by the ammoniation of phosphoric acids or slurries of solid products derived therefrom, such as, for example, monoammonium phosphate. However, only in recent years was suitable technology developed for the production, from impure phosphoric acids or satisfactory concentrated orthophosphate suspension fertilizers, by methods which overcome the effect caused by the formation of the metallic impurity gel-like compounds, which compounds normally tend to destroy the fluidity of concentrated ammonium orthophosphate suspension fertilizers. Until the development of the latest technology, the only way known for increasing the fluidity of such orthophosphate suspensions was by means of dilution of same with water, which, of course, reduced the grades thereof.
In U.S. Pat. No. 3,019,099, Walters, Jan. 30, 1962, assigned to the assignee of the present invention, produced a low-analysis (8-24-0 grade) ammonium orthophosphate suspension fertilizer by both batch- and continuoustype ammoniation of wet-process orthophosphoric acids. In the preparation of this suspension fertilizer product, Walters recognized that the impurities in the acid imparted a thixotropic characteristic causing higher grade suspension fertilizers to be excessively viscous. Walters used the gel-like impurity compounds for prevention of sedimentation in his suspension fertilizer products. Later in U.S. Pat. Nos. 3,109,729, Slack et al., Nov. 5, 1963 and 3,113,858, Slack et al., Dec. 10, 1963, both assigned to the assignee of the present invention, produced NPK suspension fertilizers in which they added both polyphosphate, for increasing the grade, and clay as both a nucleating and suspending agent. In U.S. Pat. No. 3,813,233, Kendrick, May 28, 1974, assigned to the assignee of the present invention, fully described the techniques and conditions under which ammonia can be reacted with impure wet-process orthophosphoric acid without producing the gel-like impurity compounds that previously completely destroyed the fluidity of such high-grade orthophosphate suspension products. Kendrick's teachings were based primarily on ammoniation of the impure wet-process acid in two continuous stages or steps and, through the techniques he developed, he was able to make much higher grade products (e.g., 11-39-0 versus 10-30-0) with good flow characteristics and longer static storage life than had theretofore been possible. Kendrick also made similar products by a batch-simultaneous procedure which he indicated did not work quite as well as did his continuous process. However, when products that were made by Kendrick's teachings were shipped by rail, the solid portion of the suspensions settled and packed. The resulting packed crystals held tenaciously to the bottom of the railroad tank cars. In U.S. Pat. No. 4,066,432, Jones, Jan. 3, 1978, assigned to the assignee of the present invention, found that settling of the crystals changed the composition of the fluid, and the packed crystals could not be removed from the tank by ordinary procedures, thereby greatly reducing the value of the suspension fertilizer. Using this phenomenon, Jones developed a three-stage, continuous-type process for the production of satisfactory high-analysis suspension fertilizers (e.g., 13-38-0) by the ammoniation of wet-process orthophosphoric acid under conditions wherein gel-like impurity compounds were not formed. In the materials produced by the teachings of Jones supra, the resulting crystals did not settle and pack due to the application thereto of vibrational energy such as that which occurs during shipment by rail. In U.S. Pat. No. 4,511,388, Jones et al., Feb. 27, 1984, assigned to the assignee of the present invention, developed a batch-type process for production of satisfactory high-analysis suspension fertilizers (e.g., 13-38-0) by the direct ammoniation of impure wet-process orthophosphoric acid. The process developed by Jones et al. in '388 supra consisted of leaving a heel of hot product in the reactor from a previous batch equal to at least 33 percent of the total batch volume. The hot heel served to prevent severe thickening or near solidification due to mass crystallization of monoammonium phosphate salts in the pH region of about 2 to 4 during the ammoniation step and provided the nuclei and environment in the reactor for effecting the production of metallic impurity crystals instead of the gel-like impurity compounds which normally cause complete destruction of fluidity. However, until the present time, all efforts to produce high-analysis ammonium orthophosphate (N:P.sub.2 O.sub.5) base suspensions (e.g., 12-36-0, 1.5% clay) with satisfactory physical properties by the direct ammoniation of wet-process orthophosphoric acid in simple, economical batch equipment, instead of the more expensive and more complicated batch procedure described by Jones in '388 supra, have been unsuccessful.